Converters are common electric apparatuses that are used for converting electricity from one form to another. Converters are used for rising or lowering voltage or current levels to adapt the voltage of a source to the voltage of the load.
Converters are also used in connection with photovoltaic (PV) panels for supplying the power available from the panels. The power may be supplied directly to an alternating voltage network, to a DC-voltage bus or to a power consuming load. Depending on the amount of PV cells or panels connected for supplying power, the voltage is either raised or lowered for supplying the load with appropriate voltage.
In connection with the PV systems, the converter system also keeps the operating point of the panel at its maximum power point. A PV panel has a certain operating point at which the extracted power has its maximum. A maximum power point tracker (MPPT) calculates the operating point from the measured current and voltage of the panel and gives a reference value for either output voltage or current of the panel. The control system of the converter controls the actual output value of the panel to the reference, and the maximum available power is obtained from the panel. The MPPT calculates continuously the reference value and the control system adapts to this situation enabling continuous operation in the maximum power point regardless of the varying operating conditions.
Converter topologies can be roughly divided into current fed and voltage fed converters. In voltage fed converters, the power source feeds a voltage to the input of the converter. In current fed converters, the source feeds current to the input. Due to the constant-current nature of PV panels, the optimal interfacing of the panels can be implemented by using current fed converters. A current fed converter can operate within the whole range of the UI curve of the panel from the short-circuit to open-circuit conditions. In voltage-fed converters, the operation can only be carried out at the voltages equal or higher than the maximum power point voltage.